journal of palaeogeography 4 (2015) 384e412

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Lithofacies palaeogeography of the Carboniferous and Permian in the Qinshui Basin, Shanxi Province, China

* Long-Yi Shao a, , Zhi-Yu Yang a, Xiao-Xu Shang a, Zheng-Hui Xiao a,b, Shuai Wang a, Wen-Long Zhang a,c, Ming-Quan Zheng a,d, Jing Lu a a State Key Laboratory of Coal Resources and Safe Mining, School of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China b Hunan Provincial Key Laboratory of Shale Gas Resource Utilization, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China c No.105 Geological Brigade of Qinghai Administration of Coal Geology, Xining 810007, Qinghai, China d Fujian Institute of Geological Survey, Fuzhou 350013, Fujian, China article info abstract

Article history: The Qinshui Basin in the southeastern Shanxi Province is an important area for coalbed Received 7 January 2015 methane (CBM) exploration and production in China, and recent exploration has revealed Accepted 9 June 2015 the presence of other unconventional types of gas such as shale gas and tight sandstone Available online 21 September 2015 gas. The reservoirs for these unconventional types of gas in this basin are mainly the coals, mudstones, and sandstones of the Carboniferous and Permian; the reservoir thicknesses Keywords: are controlled by the depositional environments and palaeogeography. This paper presents Palaeogeography the results of sedimentological investigations based on data from outcrop and borehole Shanxi Province sections, and basin-wide palaeogeographical maps of each formation were reconstructed Qinshui Basin on the basis of the contours of a variety of lithological parameters. The palaeogeographic Shanxi Formation units include the depositional environments of the fluvial channel, flood basin (lake), upper Taiyuan Formation delta plain, lower delta plain, delta front, lagoon, tidal flat, barrier bar, and carbonate Coal measure platform. Coalbed methane The Benxi and Taiyuan Formations are composed mainly of limestones, bauxitic Permian mudstones, siltstones, silty mudstones, sandstones, and economically exploitable coal Carboniferous seams, which were formed in delta, tidal flat, lagoon, and carbonate platform environ- ments. The Shanxi Formation consists of sandstones, siltstones, mudstones, and coals; during the deposition of the formation, the northern part of the Qinshui Basin was occu- pied mainly by an upper delta plain environment, while the central and southern parts were mainly occupied by a lower delta plain environment and the southeastern part by a delta front environment. Thick coal zones occur in the central and southern parts, where the main depositional environment was a lower delta plain. The thick coal zones of the Taiyuan Formation evidently occur in the sandstone-rich belts, located mainly in the lower

* Corresponding author. E-mail address: [email protected] (L.-Y. Shao). Peer review under responsibility of China University of Petroleum (Beijing). http://dx.doi.org/10.1016/j.jop.2015.06.001 2095-3836/© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). journal of palaeogeography 4 (2015) 384e412 385

delta plain environment in the northern part of the basin and the barrier bar environments in the southeastern part of the basin. In contrast, the thick coal zones of the Shanxi For- mation extend over the mudstone-rich belts, located in the areas of the lower delta plain environments of the central and southern parts of the Basin. The Xiashihezi, Shangshihezi, and Shiqianfeng Formations consist mainly of red mudstones with thick-interbedded sandstones. During the deposition of these formations, most areas of the basin were occupied by a fluvial channel, resulting in palaeogeographic units that include fluvial channel zones and flood basins. The fluvial channel deposits consist mainly of relatively-thick sandstones, which could have potential for exploration of tight sandstone gas. © 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

that the coal measures (Benxi, Taiyuan, and Shanxi Forma- 1. Introduction tions) were formed within delta and tidal flat-lagoon depo- sitional systems (Cheng, 1992; IGE-CCMRI/SPCEC, 1987; Shao The Qinshui Basin in southeastern Shanxi Province, northern et al., 2007). However, studies focusing on the depositional China has become a hot spot for coalbed methane exploration controls of the distribution of the No. 3 and No. 15 coal seams, and production since the 1990s (Liu et al., 1998; Zhang and the targeted seams for methane extraction, are still scarce. Wang, 1999). The coalbed methane resource in this basin is Furthermore, the facies distribution of the overlying non- 12 3 12 3 estimated to be 3.28 10 m (CNACG, 1998) or 5.52 10 m marine strata (Xiashihezi, Shangshihezi, and Shiqianfeng (Zhang et al., 2002). Recent exploration has also confirmed Formations) is not fully analyzed and the depositional that this basin has great potential for shale gas and tight mechanisms of these siliciclastic successions are still not sandstone gas in its Carboniferous and Permian strata (Dai well understood. These aspects would influence the design and Qi, 1981; Gu et al., 2011; Qin et al., 2014; Shao et al., schemes of unconventional gas exploration and extraction in 2014). Because of this large amount of unconventional re- this basin. In addition, a basin-wide synthesis of palae- sources, the Qinshui Basin has attracted extensive coalbed ogeography and its evolution for these lithological forma- methane exploration and development from a variety of tions has not been provided, and the palaeogeographical authorities, including the China United Coalbed Methane Co. evolution and its controls on the facies distribution should be Ltd., China National Petroleum Corporation, and overseas of significance to unconventional gas exploration and pro- joint ventures, as well as some local coal mine administra- duction in this basin. tions such as the Jincheng Coal Industry Group Co. The From the 1970s to the 1980s, coal depositional models 3 highest coalbed methane production is 16,000 m /d per well, established for the Carboniferous coal measures in northern e 3 and the average is 2000 4000 m /d per well in this basin in England and Appalachian basins have been proven to be recent years. Exploration has confirmed that coalbed methane effective in coal exploration in these areas (Ferm and Staub, reservoirs in this basin are dominated by semi-anthracite and 1984; Fielding, 1984; Guion et al., 1995; Horne et al., 1978; anthracite with vitrinite reflectance between 2.2% and 4.5%, Marchioni et al., 1996). Although depositional environments with high permeability and high gas contents (Jin et al., 2008; and coal accumulation have been studied in parts of the Qin- Su et al., 2005). The shale gas resource assessment organized shui Basin, e.g., the Yangquan mine area and the Shouyang ' recently by the Ministry of Land and Resources of the People s mine area (Ge et al., 1985; Huang et al., 1989), a basin-wide 12 3 Republic of China has confirmed a reserve of 0.49 10 m , depositional model and lithofacies palaeogeography of the showing a great potential for shale gas exploration in this Carboniferous and Permian have not been studied, and this basin (Fang et al., 2013; Gu et al., 2011). should be of great significance to the exploration of coal, As an important energy resource basin, the Qinshui Basin coalbed methane, as well as tight sandstone gas. has been widely investigated for its geological characteris- In this paper, we focus on basin-wide palaeogeographical tics, including the structural geology, depositional environ- reconstruction of each formation of the Carboniferous and ments, coal accumulation, coal measures stratigraphy, and Permian, based on numerous selected data points including coalbed methane geology (Chen and Liu, 1998; Qin et al., 1997; outcrop sections and boreholes. Sun et al., 1998; Su et al., 2005; Wei et al., 2002; Ye et al., 2002). In particular, the depositional models of the coal measures (Benxi, Taiyuan and Shanxi Formations) in this basin have 2. Geological setting been extensively studied (Cheng, 1992; CNACG, 1997; Ge et al., 1985; Huang et al., 1989; IGE-CCMRI/SPCEC, 1987; The Qinshui Basin is located at the southeastern Shanxi Shao et al., 2007; Zhang et al., 2001). It is generally accepted Province, with geographical coordinates of N35 e38 and 386 journal of palaeogeography 4 (2015) 384e412

Fig. 1 e Division of the mining and exploration areas in the Qinshui Basin. IeShouyang area; IIeYangquan area; IIIeHeshuneZuoquan area; IVeTunliueZhangzi area; VeGaopingeFanzhuang area; VIeJincheng area; VIIeAnzeeQinshui area; VIIIeQinyuan area. Only some representative studied outcrops and boreholes are shown with names, while others only with the location. journal of palaeogeography 4 (2015) 384e412 387

Fig. 2 e Stratigraphic subdivision and sub-correlation of the Carboniferous and Permian strata in the Qinshui Basin (modified after Cheng, 1992 and CNACG, 1997). 388 journal of palaeogeography 4 (2015) 384e412

E111000e113500. The basin is a syncline elongated in an NNE cratonic basin (Han and Yang, 1980). The boundary between direction, and its total area is approximately 30 103 km2, the Late Carboniferous and Permian is located at the base of with the width from west to east being 120 km and the length the Miaogou Limestone marker of the Taiyuan Formation from south to north being 330 km (Zhang and Wang, 1999). (Kong et al., 1996). The coal-bearing series include the Late The current Qinshui Basin is divided into the Shouyang, Carboniferous Benxi Formation, the Late Carboniferous to Yangquan, HeshuneZuoquan, TunliueZhangzi, Gao- Early Permian Taiyuan Formation, and the Early Permian pingeFanzhuang, Jincheng, AnzeeQinshui, and Qinyuan Shanxi Formation. The Benxi and Taiyuan Formations are mining and exploration areas (Fig. 1). composed of sandstone, siltstone, and mudstone, interbedded Tectonically, the Qinshui Basin was located in the eastern with typical marine bioclastic limestones, as well as coal part of the LvliangeTaihang fault block. During the Carbonif- seams, which were formed in a tidal flat-lagoon setting (Liu erous and Permian, the Qinshui Basin was part of the North et al., 1998). The Shanxi Formation consists of sandstone, silt- China Basin, which was a great cratonic depositional basin. stone, and mudstone, and mineable coal seams, which were The present Qinshui Basin is a residual structural basin that deposited in a typical delta environment. The regionally stable, was formed by continuous shearing and continuous expan- major mineable No. 15 and No. 3 coals were developed in the sion of the uplifts during the Yanshanian (Late Jurassic to Taiyuan and Shanxi Formations, respectively. The Xiashihezi Early Cretaceous) orogeny (Hsu¨ , 1989). This shearing and (Middle Permian), Shangshihezi (Middle to Late Permian), and compressive stress intensified during the Yanshanian period, Shiqianfeng (Late Permian) Formations are composed of and as a result, the great North China Basin gradually retrea- sandstones, siltstones, and typical purple mudstones, which ted towards the Ordos area to the west. The Shanxi block were formed in the fluvial system and lacustrine-dominated became an uplifted area at the middle stage of the Yanshanian system and in an arid to semi-arid climate. orogeny, accompanied by a final consolidation of the Qinshui Basin, a superimposed synclinorium (Chen and Liu, 1998). The strata of the Carboniferous and Permian include the 3. Methods Benxi, Taiyuan, Shanxi, Xiashihezi, Shangshihezi, and Shi- qianfeng Formations in ascending order (Fig. 2), which were The database for this study consists of 98 fully cored bore- generally formed in a transitional marine and non-marine holes and outcrop sections, which cover most of the Qinshui

Fig. 3 e Lithofacies of the Carboniferous and Permian in the Qinshui Basin. aeConglomerate, Taiyuan Formation (Borehole Y5-2, Taiyuan Xishan Coalfield); beTabular cross-bedded sandstone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); ceWavy cross-bedded sandstone, Taiyuan Formation (Borehole Y5-2, Taiyuan Xishan Coalfield); deMassive sandstone beds, Shanxi Formation (Borehole 7-14, Taiyuan Xishan Coalfield); eeTrough cross-bedded sandstone, Shanxi Formation (Qiligou section, Taiyuan Xishan Coalfield); feTrough cross-bedded sandstone, Shanxi Formation (Qiligou section, Taiyuan Xishan Coalfield). Booklet size ¼ 127 £ 182 mm2. journal of palaeogeography 4 (2015) 384e412 389

Basin (Fig. 1). Lithofacies were described on the basis of centers (Shao et al., 2003, 2008a, 2009; Zhang et al., 1993). depositional characteristics including lithology (hand spec- The palaeogeographic maps were mainly based on the imen and thin section), sedimentary structures, fossils, sandstone/mudstone ratio contours and limestone isopachs logging curves, and sandbody geometries. Lithofacies were and modified by other parameters such as the sandstone arranged into different depositional systems, which were isopachs and mudstone isopachs. The time intervals used described and correlated in a northesouth sedimentary for the palaeogeographical reconstruction corresponded to cross section. The analytical contour maps include the the deposition of the lithological formations, including the strata thickness, sandstone thickness, sandstone percent- Benxi Formation, Taiyuan Formation, Shanxi Formation, age, sandstone/mudstone ratio (sandstone and conglom- Xiashihezi Formation, Shangshihezi Formation, and Shi- erate/siltstone and mudstone), mudstone thickness, qianfeng Formation. limestone thickness, and total coal thickness. The isopachs of strata generally reflect the distribution of subsiding and uplifting areas, and the sandstone thickness and percent- 4. Lithofacies and depositional systems ages can indicate the main direction of the provenances and reveal the distribution of the main channels and the fluvial 4.1. Lithofacies characteristics sand bodies (Feng, 1992, 2004; Feng et al., 2014). The sand- stone/mudstone ratio can reflect the outline of the palae- The lithofacies and depositional systems of the Carbonif- ogeography and framework facies, and the isopachs of erous and Permian in the Qinshui Basin were analyzed on marine limestones can indicate the direction and scope of the basis of outcrop sections and borehole cores and in a transgression. The isopachs of coal seams can reflect the north-south sedimentary section. Using lithological and distribution of coal-forming mires and coal-accumulating palaeontological characteristics, as well as geometry and

Fig. 4 e Lithofacies of the Carboniferous and Permian in the Qinshui Basin. aeWedge cross-bedded sandstone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); beTabular cross-bedded sandstone, Taiyuan Formation (Borehole 7- 14, Taiyuan Xishan Coalfield); ceMassive sandstone beds, Shanxi Formation (Borehole 7-14, Taiyuan Xishan Coalfield); deSandstone with flaser beddings, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); eeHorizontally bedded siltstone, Taiyuan Formation (Borehole 7-2, Taiyuan Xishan Coalfield); feHorizontally bedded siltstone, Shanxi Formation (Borehole 7-14, Taiyuan Xishan Coalfield); geSandstone with flaser beddings, Shanxi Formation (Borehole 2-7, Taiyuan Xishan Coalfield); heWavy cross-bedded siltstone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield). 390 journal of palaeogeography 4 (2015) 384e412

Fig. 5 e Lithofacies of the Carboniferous and Permian in the Qinshui Basin. aeHorizontally bedded mudstone, Taiyuan Formation (Borehole Y5-2, Taiyuan Xishan Coalfield); beHorizontally-bedded mudstone, Taiyuan Formation (Borehole SX- 306, Taiyuan Xishan Coalfield); ceMassive mudstone, Taiyuan Formation (Borehole 9-2, Taiyuan Xishan Coalfield); deCarbonaceous mudstone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); eePurple and red mudstone, Taiyuan Formation (Borehole Y5-2, Taiyuan Xishan Coalfield); feMicritic limestone, Taiyuan Formation (Borehole 7-2, Taiyuan Xishan Coalfield); geMuddy limestone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); heBioclastic limestone, Taiyuan Formation (Borehole 7-14, Taiyuan Xishan Coalfield); ieBauxitic mudstone, Benxi Formation (Qiligou section, Taiyuan Xishan Coalfield); jeCarbonaceous mudstone, Shanxi (Qiligou section, Taiyuan Xishan Coalfield); keCoal, Shanxi Formation (No. 3 coal, Qiligou section, Taiyuan Xishan Coalfield).

lateral relationships, a total of 22 types of distinct lith- Conglomerate: Conglomerates are mainly fine- to ofacies were identified, and some of their characteristics are medium-grained, thin-bedded, and compositionally imma- shown in Figs. 3e5. Their salient features and probable ture (Fig. 3a). The conglomerates are less developed in the sedimentary interpretations are summarized in Table 1. Carboniferous and Permian, and they were only formed as These 22 lithofacies belong to the following 6 major litho- the channel lag deposits in some horizons of the Shang- logical types. shihezi Formation. Table 1 e Lithofacies of the Carboniferous and Permian in Qinshui Basin, Shanxi Province, China. Lithological Lithofacies Lithology Environmental interpretation types Conglomerate 1) Conglomerate Fine to medium-grained conglomerate, low maturity, thin-bedded Channel lag deposit

Sandstone 2) Trough cross-bedded sandstone Gray white and purple, thick-bedded, few gravels, with basal erosional Point bar of meandering fluvial channel, distributary channel and surface mouth bar of delta systems 3) Parallel-bedded sandstone Fine to medium-grained sandstone, gray white, medium and thick- Distributary channel and tidal flat deposit bedded, with parting lineation 384 (2015) 4 palaeogeography of journal 4) Wavy cross-bedded sandstone Grayish white and gray, thick-bedded, with animal and plant fossils, Distributary channel, tidal flat and flood plain abundant trace fossils 5) Massive sandstone beds Grayish white and gray, thick-bedded, well-sorted, angular-sub Fluvial channel deposit, delta plain distributary channel and mouth angular, with basal erosional surface and plant fossils bar 6) Wedge cross-bedded sandstone Gray and brownish gray, thick-bedded, with mudstone gravels and Delta plain distributary channel, barrier island and mouth bar plant fossils, basal erosional surface 7) Sandstone with flaser beddings Grayish white-gray black, medium and thick-bedded, well-sorted and Tidal flat well-rounded 8) Tabular cross-bedded sandstone Grayish white and gray, thick-bedded, well-sorted and well-rounded, Barrier island, mouth bar, point bar deposit and distributary channel basal erosional surface

Siltstone 9) Horizontally bedded siltstone Grayish brown-grayish black, thin-bedded, animal and plant fossils, Flood basin, interdistributary bays, delta front and lagoon with argillaceous inclusion locally 10) Massive siltstone beds Medium and thick-bedded Delta plain interdistributary bays and delta front 11) Wavy cross-bedded siltstone Gray white and gray Interdistributary bays and tidal flat

Mudstone 12) Silty mudstone Gray and gray black, with animal and plant fossils Delta plain interdistributary bays, lagoon, flood basin etc. 13) Horizontally bedded mudstone Gray and gray black, medium and thick-bedded, with animal and plant Delta plain interdistributary bays, lagoon and muddy offshore shelf fossils e

14) Massive mudstone Purple and gray black, medium and thick-bedded, with animal and Flood basin, delta front and muddy offshore shelf 412 plant fossils 15) Carbonaceous mudstone Grayish black and black Peat mire 16) Purple and red mudstone Purple and red, thick-bedded, with high proportion Flood plain 17) Bauxitic mudstone Grayish white, thick-bedded Lagoon 18) Sideritic mudstone Gray and gray black, thick bedding, with high proportion Lagoon

Limestone 19) Bioclastic limestone Grayish white, thick-bedded, abundant fossil bioclasts Carbonate platform 20) Micritic limestone Gray, thick-bedded, with animal fossils like corals Carbonate platform 21) Muddy limestone Gray, thin-bedded Carbonate platform

Coal 22) Coal Black, powdered coal primarily, thin-medium and thick-bedded Peat mire 391 392 journal of palaeogeography 4 (2015) 384e412

Sandstone: Sandstones in the research area are mainly limestone (Fig. 5g). Bioclastic limestone is grayish white and grayish white, grayish brown, and gray, and they are devel- thick-bedded, with abundant marine fossils and fossil frag- oped with a variety of cross-bedding. The sandstones can be ments. Micritic limestone is gray and thick-bedded, with coral seen in the whole successions of the Carboniferous and and fusulinid fossils. Muddy limestone is gray and thin- Permian, and they were formed in the meandering fluvial bedded, with abundant terrestrial siliciclastic clay. Lime- channel, distributary channel of the delta plain, delta front, stones are mainly developed in the Benxi and Taiyuan For- barrier bar and tidal flat environments. The lithofacies mations, and they were formed in a carbonate platform belonging to sandstones include trough cross-bedded sand- environment. stone (Fig. 3e, f), parallel-bedded sandstone, wavy cross- Coal: The main coal-forming environment in the research bedded sandstone (Fig. 3c), massive sandstone beds (Figs. 3d area was the mires formed in a tidal flat-lagoon environment and 4c), wedge cross-bedded sandstone (Fig. 4a), sandstone and an interdistributary bay environment. The coals are pri- with flaser beddings (Fig. 4d, 4g), and tabular cross-bedded marily developed in the Taiyuan and Shanxi Formation sandstone (Figs. 3b and 4b). (Fig. 5k) and less developed in the Benxi Formation. No coals Siltstone: Siltstones in the research area are mainly grayish are found in the other formations. brown and grayish black, and developed with ripple and horizontal laminations, sometimes with root traces and plant 4.2. Depositional systems fragment fossils. Siltstones are widely found in all lithological formations of the Carboniferous and Permian, and they were Twenty-two distinct lithofacies (Table 1) have been inter- mainly deposited in a low energy environment, such as the preted to have been formed in environments ranging from levee, crevasse splay, flood basin, interdistributary bay, delta piedmont fluvial channel to fully subaqueous lacustrine en- front, tidal flat, and lagoon environments. The lithofacies vironments, which collectively represent an epi-continental belonging to siltstones include horizontally bedded siltstone basin setting with transitional marine to non-marine deposi- (Fig. 4e, f), massive siltstone beds, and wavy cross-bedded tional environments. Five depositional systems are proposed: siltstone (Fig. 4h). (1) fluvial, (2) delta, (3) lacustrine, (4) tidal flat-lagoon, and (5) Mudstone: According to color, mixture composition, and carbonate platform (Table 2). Each of these depositional sys- sedimentary structures, mudstones can be subdivided into tems has its own sedimentary facies and subfacies. The dis- silty mudstone, horizontally bedded mudstone (Fig. 5a, b), tribution of these depositional systems and sedimentary massive mudstone (Fig. 5c), carbonaceous mudstone (Fig. 5d, facies in several of our representative studied outcrop and j), purple and red mudstone (Fig. 5e), bauxitic mudstone borehole sections are shown in Figs. 6e8. (Fig. 5i), and sideritic mudstone. They were normally formed in the flood basin, delta plain interdistributary bay, delta front, 4.2.1. Fluvial depositional system lagoon, and peat mire environments. Mudstones are widely Fluvial channel sandstones are laterally continuous and found in all formations of the Carboniferous and Permian. associated with well-developed overbank and flood basin de- Limestone: The limestone can be subdivided into bioclastic posits (Fig. 6a). The deposits of this system are mainly found in limestone (Fig. 5h), micritic limestone (Fig. 5f), and muddy the Shangshihezi Formation of the Qinshui Basin. The

Table 2 e Depositional systems and corresponding facies/subfacies of the Carboniferous and Permian in the Qinshui Basin. Depositional systems Sedimentary facies Subfacies Lithofaciesa Fluvial (Meandering) Flood basin Back-swamp, back-lake 9, 12, 15 Overbank Crevasse splay, levee 9, 10, 11 Fluvial channel Point bar, channel lag 1, 2, 5, 8

Delta Upper delta plain Distributary channel 2, 3, 4, 5, 6, 8 Interdistributary bay, interdistributary swamp 9, 10, 12, 15, 22 Lower delta plain Distributary channel 2, 3, 4, 5, 6, 8 Interdistributary bay, interdistributary swamp 9, 10, 12, 15, 22 Delta front Mouth bar 2, 5, 6, 8 Distal bar 10

Lacustrine Shore lacustrine 9, 11, 13 Shallow lacustrine 3, 12, 14, 15 Sand bar 5

Tidal flat-lagoon Sandy flat Tidal flat Mixed flat 3, 4, 7 Muddy flat Tidal channel

Lagoon 13, 16, 17, 18 Barrier bar 6, 8

Carbonate platform Open platform, restricted platform 19, 20, 21

a Refers to numbers of lithofacies in Table 1. journal of palaeogeography 4 (2015) 384e412 393

lenticular sandy conglomerates with erosional bases repre- crevasse splay facies, and the red and multi-colored mud- sent channel lag deposits. The channel lag deposits grade stones intercalated with thin carbonaceous mudstones of the upwards into point bar deposits, which are composed of interdistributary flood basin facies. The lower delta plain laterally continuous sand bodies with tabular cross-bedding facies is represented by the Shanxi Formation and upper part and also parallel and trough cross-bedding. The fine-grained of the Taiyuan Formation, and comprises sandstones of dis- sandstones and siltstones and intercalated reddish-colored tributary channel facies, as well as thinly interbedded carbo- mudstone layers usually make overbank deposits, which are naceous mudstones, siltstones, and sideritic sandstones of developed with ripple lamination or, more typically, climbing the interdistributary bay facies. The sandstones are developed ripple laminations. Root traces can be seen in these deposits. with large-scale cross-bedding and erosional bases associated The thinly interbedded siltstones and mudstones, in associ- with mudstone gravels and fossil tree trunks. Coal-forming ation with the sideritic concretions, indicate a flood basin mires are often well developed in the lower delta plain environment. where they initially tended to form in an interdistributary bay and subsequently developed to occupy most areas of the 4.2.2. Delta depositional system lower delta plain during periods of delta abandonment. As a Sheet-like channel sand bodies interbedded with dark-colored consequence, the lower delta plain has a typical vertical fine-grained subaqueous sediments and coals constitute an intercalation of coal, mudstone, siltstone and sandstone, overall upward-coarsening succession of this system (Fig. 6b). showing saw-like well log curves. The No. 3 coal seam with a The upper delta plain facies is represented by the Xiashihezi stable regional distribution was formed in the interdistribu- Formation in the entire Qinshui Basin, and is typically char- tary bay environments of the lower delta plain. The delta front acterized by the thick and cross-bedded sandstones of fluvial facies, in this study, represent the subaqueous parts of the channel facies, the ripple laminated siltstones of the levee and delta. The delta front comprises subaqueous, distributary

Fig. 6 e Depositional systems of the Carboniferous and Permian in the Qinshui Basin (Borehole SX-306). aeFluvial; beDelta; ceTidal flat-lagoon; deCarbonate platform. 394 journal of palaeogeography 4 (2015) 384e412

Fig. 7 e Columnar section showing sedimentary facies of the coal-bearing successions of the Carboniferous and Permian in the Shouyang area of the Qinshui Basin. journal of palaeogeography 4 (2015) 384e412 395

Fig. 8 e Columnar section showing sedimentary facies at the Qiligou section of the Taiyuan Xishan Coalfield. 396 journal of palaeogeography 4 (2015) 384e412

channel, massive to cross-bedded, calcite-cemented sand- deposits in these formations consist of carbonaceous stones, mouth bar sandstones and siltstones with planar mudstones and bauxitic mudstones which are developed cross-bedding and ripple laminations, and distal bar sheet- with horizontal laminations, or no bedding planes, and like ripple laminated siltstones. The pro-delta deposits are siderite nodules, and scarce marine fossil fragments mainly silty mudstones and thin-bedded siderite or sideritic (Fig. 6c). The sandstones interbedded with these mudstones mudstone layers with ripple or horizontal laminations. are developed with planar beds and also lenticular beds, and can be regarded as sand-rich barrier bars and tidal flat 4.2.3. Lacustrine depositional system deposits. Lacustrine deposits are well developed in the Shangshihezi and Shiqianfeng Formations and are particularly important in the 4.2.5. Carbonate platform system mudstone members of these formations. They consist of The deposits of this system are represented by limestone mainly brownish gray, and reddish mudstones with horizontal marker beds, which are well developed in the Taiyuan For- laminations. The sandstones with low-angle cross-bedding are mation (Fig. 6d). Abundant marine fossils such as fusulinids, considered to be longshore bar deposits of the coastal lake. The crinoids, and brachiopods, were found in this formation. grayish bioturbated siltstones, intercalated with mudstones, Some trace fossils of Zoophycos and Planolites can also be seen were interpreted to be bars in a shallow lake. The thick-bedded in these limestones (Hu et al., 2013). red or purple mudstones represent a fairly shallow lacustrine facies (Fig. 8; Table 1). All these lacustrine deposits can pass into 4.3. Lateral distribution of the depositional systems sandstone-rich fluvial channel belts and therefore can be attributed to the inter-fluvial lake system or back-lake facies of The north-south cross section can be used to analyze the the fluvial system. lateral distribution of sedimentary systems in the Qinshui Basin (Fig. 9). The sedimentation in the Benxi and Taiyuan 4.2.4. Tidal flat-lagoon depositional system Formations was mainly dominated by a tidal flat-lagoon The lagoonal sedimentary facies were mainly developed in depositional system and a carbonate platform depositional the Benxi and Taiyuan Formations. The typical lagoonal system. During the deposition of the Taiyuan Formation, the

Fig. 9 e North-south cross section showing sedimentary facies of the Carboniferous and Permian System in the Qinshui Basin. journal of palaeogeography 4 (2015) 384e412 397

northern zone was developed with a lower delta plain envi- Shangshihezi Formation, the Qinshui Basin was also domi- ronment, the central and south parts were dominated by a nated by a fluvial depositional system. Most of the basin was barrier bar and lagoonal environment, while the south- occupied by a flood basin, and only the northern and eastern corner was occupied by a carbonate platform. The southern parts consisted of fluvial channels. The conditions conditions for the accumulation of coal gradually became for the accumulation of coal seams were very unfavorable; unstable from the northern to southern part. During the hence, no coal seams can be found in this formation. deposition of the Shanxi Formation, most of the Qinshui basin was dominated by a delta depositional system: the northern part was mainly occupied by an upper delta plain 5. Palaeogeographic maps environment, the central part was constituted mainly of a lower delta plain environment, and the southeastern part 5.1. Lithofacies palaeogeography of the Benxi Formation was occupied by a delta front environment. Coal seams were thicker in the central and southern parts than in the northern 5.1.1. Lithofacies palaeogeography analysis of the Benxi zone. During the deposition of the Xiashihezi Formation, the Formation Qinshui Basin was mainly occupied by a fluvial depositional The palaeogeographic analysis of the lithofacies in our study system, the northern and southern parts were dominated by was mainly based on the contour map of the sandstone/ flood basins, while the central, northeastern, and central- mudstone (S/M) ratios, supported by the contour maps of southern parts were mainly occupied by fluvial channels. other individual lithological parameters such as formation As interpreted from the sedimentary environment, the con- thickness, mudstone thickness, sandstone thickness, and ditions for the accumulation of coal seams were not favor- limestone thickness. able during this period, since only thin coal seams can be The Benxi Formation in the Qinshui Basin consists of sec- found in some areas. During the deposition of the tions ranging from the basal unconformity of the Upper

Fig. 10 e Isopach map showing the total thickness of the Fig. 11 e Isopach map showing the total thickness of Benxi Formation in the Qinshui Basin. limestone of the Benxi Formation in the Qinshui Basin. 398 journal of palaeogeography 4 (2015) 384e412

e Fig. 12 e Isopach map showing the total thickness of Fig. 13 Contour map of percentages of sandstones in the sandstones in the Benxi Formation of the Qinshui Basin. Benxi Formation of the Qinshui Basin.

Carboniferous to the bottom of Jinci Sandstone in the over- aneJiexiu area in the southwestern part of the basin, indi- lying Taiyuan Formation. The Benxi Formation is composed of cating transgression of the Benxi Formation into a north- carbonaceous mudstones, limestones, conglomerates, and eastern direction (Fig. 11). The total thickness of the bauxitic mudstones. The carbonaceous mudstones are brown sandstones ranges from 0 m to 27.1 m, averaging 3.4 m, with and black in color, and can be found with or without hori- the maximum thickness being found in the zontal laminations. The conglomerates are gray to white-gray ShouyangeYangquan area near the northern part, and the and brecciated, and are rarely seen in this formation. The DuanshieMabi area in the southern part of the basin bauxitic mudstones are dense and massive and are light gray (Fig. 12). The percentages of sandstones vary between 0% to gray in color, with coaly debris and pyrite concretion on the and 68%, averaging 14%, with the highest percentage near top. the ShouyangeYangquan area and the DuanshieMabi area Within the boreholes selected for our study, the total (Fig. 13). The S/M ratios of the Benxi Formation vary be- thickness of the Benxi Formation ranges from 5 m to 52.8 m, tween 0 and 3.25, averaging 0.32, with the highest value averaging 20 m, with the maximum thickness being found near the ShouyangeYangquan area and the DuanshieMabi in the ShouyangeYangquan area (being 52.8 m) (Fig. 10). The area in the southern part of the basin (Fig. 14). From these total thickness of the Benxi Formation shows a decrease contour maps, it can be seen that two sandstone-rich zones from north to south. The total thickness of limestones existed in the Qinshui Basin. One of these sandstone-rich varies between 1.2 m and 8.8 m, averaging 4.4 m, with the zones is located in the ShouyangeYangquan area in the maximum thickness in the XiyangeHeshuneZuoquan area northern part of the Basin, and the other is located in the on the eastern edge of the basin, as well as the Fush- DuanshieMabi area. journal of palaeogeography 4 (2015) 384e412 399

Fig. 14 e Contour map of the sandstone/mudstone ratio of Fig. 15 e Lithofacies palaeogeography of the Benxi the Benxi Formation in the Qinshui Basin. Formation in the Qinshui Basin.

5.1.2. Palaeogeographic map of the Benxi Formation saw a marine transgression from the northeastern direction The palaeogeographic map of the Benxi Formation was (CNACG, 1997), it was inferred that the Qinshui Basin also had reconstructed based on the above-mentioned contour maps, a northeasterly transgression during deposition of the Benxi as well as on the analysis of the sedimentary facies in the Formation. outcrop and borehole sections (Fig. 15). The total thickness of the Benxi Formation diminishes from north to south, which 5.2. Lithofacies palaeogeography of the Taiyuan indicates that the source of the sediment was an uplifted area Formation that existed towards the southern part of the basin during the deposition of the Benxi Formation. Most areas of the Qinshui 5.2.1. Lithofacies palaeogeography analysis of the Taiyuan Basin were dominated by a lagoonal environment, which is Formation manifested by the basin-wide distribution of the mudstones. The Taiyuan Formation in Qinshui Basin consists of sections The barrier bar environments were indicated by the two from the base of the Jinci Sandstone to the bottom of Bei- sandstone-rich zones, near the northeastern part around the chagou Sandstone of the Shanxi Formation. The Taiyuan ShouyangeYangquan area and in the southern part around Formation is composed of limestones, fine to medium-grained the DuanshieMabi area, respectively. The limestone-rich sandstones, siltstones, silty mudstones, carbonaceous mud- zones, represented by the ZuoquaneHeshuneXiyang area stones, and coal seams. The limestone markers, namely the and the FushaneJiexiu area, denote the carbonate platform “Miaogou”, “Maoergou” and “Fucheng” Limestones, are mainly environments. Considering the Late Carboniferous bioclastic limestones, which were widely developed with palaeogeography of the great northern China Basin, which marine fossils such as brachiopods, fusulinids, and crinoids 400 journal of palaeogeography 4 (2015) 384e412

e Fig. 16 e Isopach map showing the total thickness of the Fig. 17 Isopach map showing the total thickness of Taiyuan Formation in the Qinshui Basin. limestones of the Taiyuan Formation in the Qinshui Basin.

averaging 104.6 m, with the maximum thickness in some (IGE-CCMRI/SPCEC, 1987), as well as trace fossils such as Zoo- boreholes in the Shouyang area being 207.4 m (Fig. 16). Fig. 16 phycos and Rhizocorallium (Hu et al., 2013). These characteris- also shows a trend of decreasing thickness from the tics suggest that the limestones were mainly deposited in the TaiyuaneShouyangeZuoquan area to the southwest and from carbonate platform environment. The gray mudstones, silt- the Qinyuan and JinchengeGaopingeZhangzi area to the stones and sandstones in this formation contain abundant middle of the TunliueMabi area in the central and southern lenticular, nodular, or bedded siderites and sideritic mud- parts. A total of 3 thicker zones can be identified from the stones, developed with horizontal bedding and lenticular isopach map of the Taiyuan Formation, which include the bedding, as well as burrows. These characteristics indicate TaiyuaneShouyangeZuoquan area, the Qinyuan area, and the that these mudstones, siltstones, and sandstones were JinchengeGaopingeShouyangeYangquan area. This forma- formed in a lagoonal setting. The medium- to fine-grained tion is generally thinner in the southern part of the basin and quartz sandstones in the Taiyuan Formation have a high thicker in the northern part, with the central part being the compositional maturity and a moderate-to-good sorting, thinnest. commonly developed with large-scale cross-bedding, espe- The total thickness of limestones varies from 0 m to 28 m, cially low-angle cross-bedding. These sandstones normally averaging 14 m, with the greatest thickness in the Gao- develop with an upward-coarsening succession and are pingeJincheng area, indicating that the transgression came believed to be tidal flat sand bar deposits. In summary, it can from southeast during the deposition of the Taiyuan Forma- be concluded that the Taiyuan Formation represents carbon- tion (Fig. 17). The coal in this formation has a total thickness ate platform, lagoonal, and tidal flat environments. from 0.7 m to 17.3 m, averaging 7.1 m, with the thickest coal Within the selected boreholes, the total thickness of the distributing around the Yangquan area in the north of the Taiyuan Formation varies between 29 m and 207.4 m, Qinshui Basin (Fig. 18). The total thickness of sandstones journal of palaeogeography 4 (2015) 384e412 401

Fig. 18 e Isopach map showing the total thickness of coals Fig. 19 e Isopach map showing the total thickness of of the Taiyuan Formation in the Qinshui Basin. sandstones of the Taiyuan Formation in the Qinshui Basin.

varies between 4.5 m and 81.1 m, averaging 27 m, with the has generally lower S/M ratios, except the northern side of the maximum thickness in the TaiyuaneShouyangeYangquan Qinyuan and GaopingeJincheng areas where the S/M ratios area (Fig. 19). These sandstone isopach maps reveal an are higher than 0.4. northerly provenance for the Taiyuan Formation, which is in consistency with an overall northern Yinshan 5.2.2. Palaeogeographic map of the Taiyuan Formation palaeocontinent provenance for the North China Platform Based on the above contour maps, such as the contours of S/M during the Carboniferous and Permian (CNACG, 1997). The ratios, the thickness of limestones, and other individual lith- percentages of sandstones range between 5% and 56%, aver- ological parameters, as well as on analysis of the sedimentary aging 24%, with the highest percentage in the Taiyuane- facies in the outcrop and borehole sections, a palaeogeo- ShouyangeYangquan area (Fig. 20) showing a similar trend graphic map of the Taiyuan Formation was reconstructed with the isopachs of the sandstones. (Fig. 22). It can be seen that the northern zone of the Qinshui The contours of the S/M ratios of the Taiyuan Formation Basin (PingyaoeQixianeTaiyuaneShouyangeYang show that there are two sandstone-rich zones in the north and quaneZuoquan area) was occupied by a lower delta plain south of the Basin, respectively (Fig. 21). The northern environment with S/M ratios higher than 0.5, the Hen- sandstone-rich zone is mainly located to the north of Qinxian, glingeWuxiangeQinxian area was occupied by the delta front including the Pingyao, Taigu, Shouyang, Yangquan, and environment with S/M ratios lower than 0.5, the central part of Xiyang areas. The S/M ratios in this zone are commonly higher the Qinshui Basin (QinyuaneTunliueZhangzi area) was than 0.4, with the maximum ratio up to 1.0 in the Shouyang developed as a tidal flat environment with S/M ratios higher and Heshun areas, except the narrow area along the Qin- than 0.5, the southern part (QinshuieMabieAnzeeFushan xianeWuxiangeYushe area in which the S/M ratios are lower area) was developed as a lagoonal environment with S/M ra- than 0.4. In contrast to the northern zone, the southern zone tios lower than 0.5, and the most southeastern corner 402 journal of palaeogeography 4 (2015) 384e412

Fig. 20 e Contour map of percentages of sandstones in the Fig. 21 e Contour map of the sandstone/mudstone ratio of Taiyuan Formation of the Qinshui Basin. the Taiyuan Formation in the Qinshui Basin.

(GaopingeJincheng area) was dominated by a carbonate platform environment with a limestone thickness of more formation is composed of fine-to medium-to coarse-grained than 20 m. The locally high S/M ratios (higher than 0.5) in the sandstone, siltstone, mudstone, and coal seams, with locally southeastern area represent a barrier bar environment developed conglomerate and limestone. The Beichagou

(Fig. 21). As the limestones are mainly distributed in the Sandstone (K7 Sandstone), one of the marker beds, was southeastern part of the Qinshui Basin, it can be inferred that developed at the bottom of this formation, and it is the transgression was from the southeastern direction, which composed of medium-to coarse-grained sandstones and was related to the regional transgression from the Huainan partially by conglomerates. Tabular and trough cross- and Huaibei areas in the great northern China Basin (CNACG, bedding and wedge-shaped cross-bedding were developed 1997; Shao et al., 2014). The sandstones gradually become in this sandstone, with basal lenticular conglomerate lags thinner form north to south, indicating a northerly prove- containing mud pebbles and fossil wood. This sandstone nance from the regional Yinshan palaeocontinent (CNACG, represents an incised valley fill deposit formed during a 1997; Shao et al., 2014). regionally lowstand distributary channel incision (Shao et al., 2008b). The deposits overlying the Beichagou Sand- 5.3. Lithofacies palaeogeography of the Shanxi stone consist of fine-grained sandstone, siltstone, Formation mudstone, and coal seams, representing a complex of an interdistributary bay environment in a lower delta plain 5.3.1. Lithofacies palaeogeography analysis of the Shanxi setting. The fine- and medium-grained sandstones in Formation several parts are moderateewell sorted with a low content The Shanxi Formation in the Qinshui Basin lies below the of mud matrix, representing a mouth bar deposit of a delta Luotuobozi Sandstone of the overlying Xiashihezi Formation front facies where sands had been repetitively winnowed by and extends to the bottom of the Beichagou Sandstone. This waves and tidal currents. journal of palaeogeography 4 (2015) 384e412 403

Fig. 22 e Lithofacies palaeogeography of the Taiyuan Fig. 23 e Isopach map showing the total thickness of Formation in the Qinshui Basin. thickness of the Shanxi Formation in the Qinshui Basin.

S/M ratios for the Shanxi Formation (Fig. 27), it can be seen In selected boreholes, the total thickness of the Shanxi that a sandstone-rich zone existed towards the north of the Formation varies between 20.2 m and 150.3 m, averaging 60 m, basin. The S/M ratios in this zone are generally higher than with the greatest thickness at the Qinxian area across the 0.5, except in a narrow belt along the QinxianeWuxiang central part of the basin (Fig. 23). The total thickness of coal area near Yangquan. In the central area of the Qinshui seams varies from 0.3 m to 10 m, averaging 4.6 m, with the Basin, a sandstone-rich zone occurs near the greatest thickness at the GaopingeDuanshi and Qinyuan QinyuaneXiangyuan area where the S/M ratios were mostly areas in the southern Qinshui Basin (Fig. 24). The total thick- lower than 0.5. In contrast to the northern zone, the ness of sandstones varies between 0.6 m and 56.4 m, aver- southern zone has generally lower S/M ratios, except in a aging 18 m, with the greatest thickness at the narrow belt along the northwestern side of the Gao- QixianeYucieSongta area in the northern Qinshui Basin pingeJincheng area. (Fig. 25). Percentages of sandstones range from 0% to 64%, averaging 29%, with the highest values at the Taiyuane- 5.3.2. Palaeogeographic map of the Shanxi Formation ShouyangeYangquan area in the northern basin, followed by A palaeogeographic map of the Shanxi Formation was con- the Xiangyuan and Gaoping areas at the southern end of the structed mainly based on the contours of S/M ratios and basin (Fig. 26). combined with other single lithological parameters, as well as S/M ratios of the Shanxi Formation vary from 0 to 2.2, on a sedimentary facies analysis of the outcrop and borehole averaging 0.6, with the highest value at the Taiyuane- sections (Fig. 28). During deposition, the northern sandstone- ShouyangeYangquan, Xiangyuan, and Gaoping areas rich zone was dominated by an upper delta plain environ- (Fig. 27), showing a similar distribution to the area with the ment, where the S/M ratios were higher than 0.5. The central highest percentage of sandstones. From the contours of the zone mainly developed as a lower delta plain environment, 404 journal of palaeogeography 4 (2015) 384e412

Fig. 24 e Isopach map of coal thickness of the Shanxi Fig. 25 e Isopach map showing the total thickness of Formation in the Qinshui Basin. sandstones of the Shanxi Formation in the Qinshui Basin.

where the S/M ratios were lower than 0.5. The southern zone developed as a delta front environment, where the S/M ratios laminations. The fine-grained sandstones are light gray, were less than 0.4. The relative enrichment of sandstones and moderately sorted, and show cross-bedding in the upward- mudstone in the northern and southern basins, respectively, fining successions. The sand grains are mainly quartz and indicates a northerly provenance, which is in accordance with feldspar with minor rock fragments with a siliceous cement. the overall provenance of the northern Yinshan palae- The mottled mudstone is typically developed at the top of ocontinent, and a southeasterly transgression direction the formation and is characterized by a mottled mixture of (CNACG, 1997). red and green colors. In selected boreholes, the total thickness of the Xiashihezi Formation varies from 26.4 m to 277.5 m, averaging 85 m, and 5.4. Lithofacies palaeogeography of the Xiashihezi shows a declining trend from west to east, with the greatest Formation thickness at the MabieAnze and PingyaoeQixian areas along the western side of the basin. This indicates that the 5.4.1. Lithofacies palaeogeography analysis of the Xiashihezi subsidence rate of the western part was higher than the Formation eastern part during the deposition (Fig. 29). The total thickness The Xiashihezi Formation in the Qinshui Basin is marked of the sandstone varies between 3.5 m and 98.3 m, averaging at the base by the bottom of the Luotuobozi Sandstone and 27.7 m, with the greatest thickness near the Shouyang, Pin- at the top by the Taohua (mottled) Mudstone (Fig. 2). gyaoeQixian, Qinyuan, and MabieAnze areas. The percent- This formation is composed of fine-to medium-grained ages of sandstones vary between 2% and 94%, averaging sandstones, siltstones, and sandy and mottled mudstones. 34.2%. Sandstones with relatively high percentage and thick- The siltstones are gray to black gray, commonly contain ness are also found in these areas. S/M ratios of the Xiashihezi mica as well as fossil plant fragments, and have ripple Formation vary from 0.02 to 1.9, averaging 0.58. S/M ratios journal of palaeogeography 4 (2015) 384e412 405

Fig. 26 e Isopach map showing the percentages of Fig. 27 e Isopach map of the sandstone/mudstone ratio of sandstones of the Shanxi Formation in the Qinshui Basin. the Shanxi Formation in the Qinshui Basin.

higher than 0.6 can define three sandstone-rich zones. The basin was dominated by littoral and shallow lacustrine en- northern zone was small and only restricted to the vironments. A northerly provenance and a southeasterly ShouyangeYangquaneXiyang area, the central zones were transgression direction were established in accordance with relatively wider, and represented by the QinyuaneXiang the overall palaeogeography of the greater North China Basin yuaneZuoquan area and the DuanshieGaopingeJincheng during the Carboniferous and Permian (CNACG, 1997; Shao areas (Fig. 30). et al., 2014).

5.4.2. Palaeogeographic map of the Xiashihezi Formation 5.5. Lithofacies palaeogeography of the Shangshihezi A palaeogeographic map of the Xiashihezi Formation was Formation reconstructed on the basis of contours of S/M ratios and other lithological parameters as well as on a sedimentary 5.5.1. Lithofacies palaeogeography analysis of the facies analysis on the outcrop and borehole sections (Fig. 31). Shangshihezi Formation During deposition the area was mainly dominated by a The Xiashihezi Formation in the Qinshui Basin is marked at fluvial channel environment, where the areas with S/M ratios the base by the bottom of the Luotuobozi Sandstone and at the higher than 0.6 were developed as fluvial channel zones, and top by the Taohua (mottled) Mudstone. other areas were mainly dominated by a flood basin envi- The Shangshihezi Formation is marked at the base by the K8 ronment with S/M ratios lower than 0.6. The fluvial channel Sandstone (overlying the Taohua Mudstone) and at the top by zones developed in the northern part around the the bottom of K Sandstone. This formation is composed of red e 13 Shouyang Yangquan area, the central part around the Zuo- and purple-colored, interbedded sandstones and mudstones. e e e quan Tunliu Qinyuan Qinxian area, and the southeastern According to its lithological characteristics, this formation can e e part around the Mabi Duanshi Jincheng area. The flood be subdivided into three parts. The lower part mainly consists of 406 journal of palaeogeography 4 (2015) 384e412

Fig. 28 e Lithofacies palaeogeography of the Shanxi Fig. 29 e Isopach map showing the total thickness of the Formation in the Qinshui Basin. Xiashihezi Formation in the Qinshui Basin.

an intercalation of grayish green and red patchy mudstone and between 11% and 65%, averaging 32%, with the highest grayish white sandstone, with the grayish white, thick-bedded, values at the ShouyangeSongtaeXiyang area north of the coarse-grained feldspathic quartz sandstones occurring at the basin and the GaopingeJincheng area southeast of the bottom. The middle part comprises grayish yellow and partly basin. S/M ratios in the ShouyangeYangquaneXiyang and purple mudstone and sandstone intercalations, with massive, the ChangzhieGaopingeJincheng areas vary between thick, grayish white sandy conglomerates and coarse-grained 0.12 and 1.86, averaging 0.5, and there are two sandstone- sandstones at its bottom. The upper part is mainly composed rich zones in the basin with S/M ratios higher than 0.6 of purple mudstones, fine-grained sandstones, and grayish (Fig. 33). yellow medium-grained sandstones, with grayish white pebbly sandstones at its bottom. The depositional environments of the 5.5.2. Palaeogeographic map of the Shangshihezi Formation Shangshihezi Formation were mainly braided and meandering A palaeogeographic map of the Shangshihezi Formation was fluvial environments. reconstructed on the basis of contours of S/M ratios and other In selected boreholes, the total thickness of the Shang- single lithological parameters as well as on a sedimentary shihezi Formation varies from 122.1 m to 740 m, averaging facies analysis of outcrop and borehole sections (Fig. 34). 385 m, with a southward thickening trend. The greatest During deposition the area was mainly dominated by fluvial thickness was found at the YucieSongta and Tunliue channel environments, including fluvial channel zones with GaopingeMabi areas (Fig. 32). The total thickness of the S/M ratios higher than 0.6 and flood basin (mainly lacustrine) sandstones varies between 31 m and 376.2 m, averaging zones with S/M ratios lower than 0.6. The fluvial channel 124 m, with the greatest thickness at the Gaoping area in the zones were mainly located in the QingxueHenglingeZuoquan south of the basin. The percentages of sandstones vary area in the northeast and the ChangzhieGaopingeJincheng journal of palaeogeography 4 (2015) 384e412 407

Fig. 30 e Isopach map of the sandstone/mudstone ratio of Fig. 31 e Lithofacies palaeogeography of the Xiashihezi the Xiashihezi Formation in the Qinshui Basin. Formation in the Qinshui Basin.

area in the southeast. A northerly provenance is inferred in The total thickness of the Shiqianfeng Formation varies from accordance with the overall palaeogeography of the greater 83 m to 286 m, averaging 186 m, with a southward thickening North China Basin during the Carboniferous and Permian trend, and the greatest thickness is in the YucieSongtaeMabi (CNACG, 1997; Shao et al., 2014). area in the north (Fig. 35). The total thickness of the sand- stones varies between 12 m and 111.9 m, averaging 61 m, 5.6. Lithofacies palaeogeography of the Shiqianfeng with the greatest thickness in the MabieDuanshie Formation GaopingeZhangzi area. The percentages of sandstones vary between 8% and 70%, averaging 39.8%. The areas with the 5.6.1. Lithofacies palaeogeography analysis of the highest percentage of sandstones occur in the ChangzhieZ- Shiqianfeng Formation hangzieMabi area in the southeast, as well as in the north- The Shiqianfeng Formation is marked at the base by the bot- eastern part of the ShouyangeHeshun area in the north. S/M tom of the K13 Sandstone and at the top by the bottom of the ratios of the Shiqianfeng Formation vary between 0.09 and

Liujiagou Sandstone (K14 Sandstone). This formation contains 2.37, averaging 0.86, and two sandstone-rich zones with S/M purple mudstone with calcareous concretions, interbedded ratios higher than 0.8 occur in the ShouyangeHeshun area in yellowish green medium-grained sandstone, purple the northeastern part and the ChangzhieZhangzieMabi area mudstone, and thick-bedded medium-to coarse-grained in the southeastern part (Fig. 36). sandstone. This formation was mainly deposited in a fluvial lacustrine environment (CNACG, 1997). 5.6.2. Palaeogeographic map of the Shiqianfeng Formation Because boreholes in the Shiqianfeng Formation are A palaeogeographic map of the Shiqianfeng Formation was scarce, the analysis was based on only 15 selected boreholes. reconstructed on the basis of contours of the S/M ratios and 408 journal of palaeogeography 4 (2015) 384e412

Fig. 32 e Isopach map showing the total thickness of the Fig. 33 e Isopach map of the sandstone/mudstone ratio of Shangshihezi Formation in the Qinshui Basin. the Shangshihezi Formation in the Qinshui Basin.

other lithological parameters, as well as on a sedimentary facies carbonate platform environments in the Benxi and Taiyuan analysis in outcrop and borehole sections (Fig. 37). During Formations, fluvial-dominated delta environments in the deposition, the area was mainly dominated by fluvial channel Shanxi Formation, and fluvial and flood basin (back-lake) to flood basin (mainly lacustrine) environments. Areas with S/M environments in the Xiashihezi, Shangshihezi, and Shi- ratios higher than 0.6 represent fluvial channel zones, whereas qianfeng Formations. The palaeogeographical patterns areas with S/M ratios lower than 0.6 consist mainly of controlled reveal an overall upward-shallowing succession where a by inter-fluvial lacustrine environments. The fluvial channel marine transgression switched from the northeast (Benxi zones developed to the east of the ShouyangeSongtae and basal Taiyuan formations) to the southeast (Taiyuan, HenglingeWuxiangeTunliu area and to the north of the Fush- Shanxi, Xiashihezi, Shangshihezi, and Shiqianfeng aneMabieChangzhi area (Fig. 37). These channel zones have a Formations). distribution that is almost the same as that of the Shangshihezi The Benxi, Taiyuan, and Shanxi Formations are dominated Formation. Therefore, the provenance is interpreted to be by dark-colored carbonaceous mudstones, gray siltstones, similar to that of the regional Yinshan palaeocontinent to the gray limestones, bauxitic mudstones, and coals, which are north. typical indicators of humid and warm climatic conditions. In contrast, red-colored mudstones commonly occur in the Xiashihezi, Shangshihezi, and Shiqianfeng Formations, rep- 5.7. Palaeogeographic evolution and fossil energy resenting semi-arid to arid climates. It is obvious that the resources palaeoclimates during the Carboniferous and Permian varied from humid to semi-arid and arid conditions. The warm and It can be concluded from the palaeogeographical analyses humid climates during the Late Carboniferous and that the Qinshui Basin represents tidal flat-lagoon and journal of palaeogeography 4 (2015) 384e412 409

Fig. 34 e Lithofacies palaeogeography of the Shangshihezi Fig. 35 e Isopach map showing the total thickness of the Formation in the Qinshui Basin. Shiqianfeng Formation in the Qinshui Basin.

every depositional system within the basin, the inter- EarlyeMiddle Permian were favorable to the development of distributary bay of the delta system and the tidal flat- coal-forming mires and accumulation of thick coal seams. lagoon system provided favorable environments for coal These coal seams not only provide abundant coal resources formation. but also abundant coalbed methane resources. The dark- Lithofacies palaeogeographic maps were reconstructed on colored mudstones in these formations can be potential tar- the basis of a number of contour maps and sedimentary facies gets for shale gas exploration (Gu et al., 2011). Sandstones in analyses. Palaeogeographical units developed during the the entire Carboniferous and Permian successions can be Carboniferous and Permian include flood basin, fluvial chan- ideal reservoirs for natural gas, providing potential explora- nel, upper delta plain, lower delta plain, delta front, lacustrine, tion sites for tight sandstone gas. tidal flat, lagoon, barrier bar, and carbonate platform facies. During the Late Carboniferous (Benxi Formation) and late Late Carboniferous to early Early Permian (Taiyuan Formation), the 6. Conclusions Qinshui Basin area experienced cyclic changes from carbon- ate platform to lagoon-delta environments, forming charac- There were five major depositional systems in the Qinshui teristic coal-bearing limestone-siliciclastic successions. Basin during the Carboniferous and Permian, including During the late Early Permian (Shanxi Formation), a delta fluvial depositional, delta, lacustrine, tidal flat-lagoon, and system was developed which was dominated by a lower delta carbonate platform systems. A total of 22 lithofacies have plain environment. been recognized in the coal-bearing successions. Despite the During the early Middle Permian (Xiashihezi Formation) to fact that coal-forming swamps could develop in almost the late Middle to early Late Permian (Shangshihezi Forma- 410 journal of palaeogeography 4 (2015) 384e412

Fig. 36 e Isopach map of the sandstone/mudstone ratio of Fig. 37 e Lithofacies palaeogeography of the Shiqianfeng the Shiqianfeng Formation in the Qinshui Basin. Formation in the Qinshui Basin.

may be potential sites for the exploration of tight sandstone gas in the future. tion) and late Late Permian (Shiqianfeng Formation), the Qinshui Basin area was dominated by a fluvial setting and an arid to semi-arid palaeoclimate, which is indicated by the intercalation of thick sandstone beds and thick red-colored Acknowledgements mudstones. The sedimentary provenance of the Basin during the Late Carboniferous was from the east and northeast, while This research was supported by the National Natural Science during the Permian it was mainly from the regional Yinshan Foundation of China (No. 41572090, No. 41472131), and the Palaeocontinent to the north. Major National S&T (Science and Technology) Program of From this study, it can be postulated that palaeoenviron- China (2016ZX05041004-003). We would like to thank Profes- ments suitable for substantial and prolonged coal accumu- sor Peng-Fei Zhang and Professor Huan-Jie Liu for their lation occurred in the entire Basin, but different encouragement. We also thank Professor Zeng-Zhao Feng and developments existed in the Taiyuan and Shanxi Formations. Professor A. J. (Tom) van Loon for their constructive com- The No. 15 coal of the Taiyuan Formation formed in tidal- ments to improve the manuscript. lagoon settings and developed thicker towards the northern part of the basin. In contrast, the No. 3 coal of the Shanxi Formation formed in a fluvial-delta setting, and developed references thicker towards the southern part of the basin. The No. 15 and No. 3 coals form sound reservoirs for coalbed methane exploration and development, and the dark-colored mud- Chen, G., Liu, L.Y., 1998. The analysis on the trap conditions of the stones and silty mudstones can be potential targets for shale Paleozoic natural gas in the Qinshui Basin. Northwest. Geol. 19 gas exploration. The sandstone beds in all these formations (4), 34e38 (in Chinese). journal of palaeogeography 4 (2015) 384e412 411

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